1. Field of the Invention
The present invention relates to a system for measuring chromatic dispersion of an optical fiber and, more particularly, to a system for measuring chromatic dispersion of an optical fiber by a baseband phase comparison method.
2. Description of the Related Art
The chromatic dispersion characteristics of optical fibers are important for determining an information transmission speed of an optical fiber communication path. The following are the main methods used at present for measuring chromatic dispersion in single-mode optical fibers:
(1) A pulse delay time difference measurement method using a fiber Raman laser/spectroscope combination;
(2) A baseband phase comparison method using an LED (light-emtting diode)/spectroscope combination;
(3) A baseband phase comparison method using multiple LDs (laser diodes) of different wavelengths; and
(4) An interference method using the interference characteristics of light.
Of these methods, the baseband phase comparison method (3) will be described below. In general, optical signals having different wavelengths differ in group velocity due to material dispersion and waveguide dispersion, resulting in phase differences of the optical signals after their propagation through an optical fiber. The baseband phase comparison method utilizes this fact. Optical signal generators such as LDs having different wavelengths are arranged in an optical signal transmitter, and two types of optical signals, i.e., a reference optical signal and a measurement optical signal, which are intensity-modulated by a sine wave modulation signal, are generated. The two types of optical signals are incident on a reference optical fiber and a measurement optical fiber, respectively. In an optical signal receiver, a group delay time difference is calculated from a phase difference between the wavelengths after propagation through the measurement optical fiber. The measurement result is approximated by appropriate function .tau.(.lambda.), and the function is analytically differentiated to obtain target chromatic dispersion characteristics D(.lambda.)=d.tau.(.lambda.)/d.lambda.. The graph in FIG. 2 represents the relationship between the group delay time difference and the wavelengths.
However, in the conventional chromatic dispersion measuring system using the baseband phase comparison method described above, a special-purpose light source, e.g., an LD is separately arranged in order to obtain the reference optical signal in addition to the measurement optical signal generators. This fact is disadvantageous in realizing a compact measurement apparatus. In addition, since a very expensive LD and associated elements must be additionally used, this interferes with reduction in cost.
Furthermore, the optical signal receiver of the conventional chromatic dispersion measuring system has a photoelectric conversion section for converting optical signals emerged from the measurement fiber and the reference fiber into electrical signals. Thus, the conventional system has a function of converting an optical signal within a predetermined level range into an electrical signal with good linearity but cannot covert the optical signal exceeding the predetermined level range into an electrical signal with good linearity. For this reason, when the optical signal level is expected to change greatly, a maunal optical attenuator or the like is provided to the input side of the photoelectric conversion section to keep the optical signal level within a predetermined range.
Such a manual attenuation value adjusting method is not satisfactory in terms of a measurement time. A loss caused by a difference between the lengths of fibers to be measured or by a coupling state varies. Therefore, when an unexpected change in optical signal level occurs, it is difficult to appropriately adjust the optical signal level. And this leads to degradation of light receiving elements.